Which system do you have, Jose - The "factory" with a cylindrical compressor or the "dealer" with one that looks like a mower engine?
We had "dealer" and though it was a pain in several ways, it blew COLD after conversion and I would not have wanted to do without it. Our Holiday on 83 E350 was 24-ft but we never ran roof air on the road. Only the dash and it cooled the whole rig. Vehicle A/C is rated in the 20,000 range and your rooftop is probably 13,500. Whenever we try to run roof air for travel, it just doesn't satisfy. You want the air in your face, not behind your back.
You need new oil, the 134 of course, new drier, fittings and label, plus a HPCO switch (High Pressure Cut Off). No need to replace hoses or O-rings unless they're failing. If it's factory, new orifice tube and new LPCO switch (or adjust the old one). If dealer, new expansion valve.
Either way don't overcharge. Start at 60% of OEM R12 charge and see what it does. Your low side may wind up low, but if you try to raise it you may find the high side goes too high. Conversions tend to jump into excess high side pressure.
